24 THE HUMAN STERNUM 



respectively). In thirty-eight cases (26-9 per cent.) there is a fourth centre 

 for the ossification of the lowest part of the mesosternum, between the 

 attachments of the fifth and seventh costal cartilages. This element receives 

 laterally the attachment of the sixth costal cartilages. 



The percentage of cases in which a fourth centre of ossification 

 occurs is lower than in the foetal sterna examined {29 per cent.), probably 

 on account of fusion in some cases of the two lower centres. There is, 

 among the cases examined, no evidence of the occurrence of a distinctly 

 segmental fifth centre for the ossification of the part of the mesosternum 

 between the sixth and seventh costal cartilages. 



Median and Bilateral Ossification of the Mesosternum. 



In the case of each so-called segment of the mesosternum, it is much 

 more common (Table VII) for ossification to occur by means of single, 

 median centres, than by two or more lateral or vertical centres. When 

 more than one centre of ossification are present, it is further much more 

 common to find them lateral (or oblique) than vertical in position. The 

 third piece of the mesosternum is the one most frequently bilateral, and 

 the first is most often single and median (PL V, Figs. 38 and 39). 



In this respect again confirmation is given to the conclusions drawn 

 from the examination of foetal sterna. The proportion of single centres in 

 sterna after birth is, doubtless, higher, on account of pre-natal fusion of 

 certain additional centres. 



Relation of Ossification of Mesosternal Elements to Asymmetry of the Sternum. 



Many instances have been noted of obliquity in the lines of junction, 

 irregularity in the form and shape, obliquity in position, of the ossifying 

 elements of the mesosternum. These irregularities have doubtless some 

 influence in causing curvature and asymmetry of the sternum, if only by the 

 approximation or separation of adjacent costal cartilages. 



Fusion of the parts of the Mesosternum. 



The number of sterna examined gives suflScient data for the deter- 

 mination of the approximate order and time of the fusion of the component 



